In wireless communication systems such as Wideband Code Division Multiple Access (WCDMA; registered trademark), Long Term Evolution (LTE), and LTE-Advanced (LTE-A) based on Third Generation Partnership Project (3GPP), and a wireless LAN or Worldwide Interoperability for Microwave Access (WiMAX) based on the Institute of Electrical and Electronics Engineers (IEEE), a base station (a cell, a transmission station, a transmission apparatus, or eNodeB) and a terminal (a mobile terminal, a reception station, a mobile station, a reception apparatus, or User Equipment (UE)) include a plurality of transmission and reception antennas, respectively, and data signals are spatially multiplexed using a Multi Input Multi Output (MIMO) technology so as to achieve high-speed data communication.
In such wireless communication systems, in a case where the base station transmits downlink data (a transport block for a downlink shared channel (DL-SCH)) to the terminal, the base station multiplexes and transmits demodulation reference signals (also be referred to as DMRS) that are signals known between the base station and the terminal. Here, the demodulation reference signals are also referred to as user equipment-specific reference signals (UE-specific RS; terminal-specific RS).
For example, the DMRS is multiplexed with the downlink data before a precoding process is applied. Therefore, the terminal can measure an equalization channel including the applied precoding process and a channel state using the DMRS. That is, the terminal can demodulate the downlink data even when the base station does not notify the terminal of the applied precoding process.
Here, the downlink data is mapped to a physical downlink shared channel (PDSCH). That is, the reference signal is used for demodulation of the PDSCH. Further, for example, the reference signal is transmitted only in a resource block (physical resource block; also referred to as resources) to which the corresponding PDSCH is mapped.
Here, a wireless communication system using a heterogeneous network deployment (HetNet) based on, for example, a macro base station having a wide coverage and an Remote Radio Head (RRH) having a narrower coverage than the macro base station has been studied. FIG. 9 is a schematic diagram of the wireless communication system using the heterogeneous network deployment. As illustrated in FIG. 13, for example, the heterogeneous network includes a macro base station 901, an RRH 902, and an RRH 903.
In FIG. 14, a macro base station 1401 builds coverage 1405, and an RRH 1402 and an RRH 1403 build coverage 1406 and coverage 1407, respectively. Further, the macro base station 1401 is connected to the RRH 1402 via a line 1408 and connected to the RRH 1403 via a line 1409. Accordingly, the macro base station 1401 can transmit or receive a data signal or a control signal (control information) to or from the RRH 1402 and the RRH 1403. Here, for example, a wired line such as an optical fiber or a wireless line using a relay technology is used for the line 1408 and the line 1409. In this case, some or all of the macro base station 1401, the RRH 1402, and the RRH 1403 use the same resources, and thus, it is possible to improve general frequency use efficiency (transmission capacity) in an area of the coverage 1405.
Further, in a case where a terminal 1404 is located within the coverage 1406, the terminal 1404 can perform single-cell communication with the RRH 1402. Further, in a case where the terminal 1404 is located near an edge (cell edge) of the coverage 1406, measures to solve interference of the same channel from the macro base station 1401 are necessary. Here, a method of reducing or suppressing the interference with the terminal 1404 in a cell edge region by performing cooperative communication between base stations to cooperate with each other between adjacent base stations has been examined as a multi-cell communication (cooperative communication) between the macro base station 1401 and the RRH 1402. For example, a Cooperative Multipoint (CoMP) transmission system has been studied as a scheme for reducing or suppressing interference due to inter-base-station cooperative communication (NPL 1).